It is known that an image may be projected on the rear surface of a translucent projection screen and viewed from the front of the screen. The contrast and definition of the image may be reduced, due to polychromatic scintillations which produce a multicolor-shimmering effect.
It has been suggested that the polychromatic shimmering of a displayed image may be eliminated by rotating a diffusion plate of a projection screen. However, the angular velocity of the plate in the suggested system is undesirably high. In the German patent application No. OS 19 45 486, it is suggested that two diffusion plates may be arranged in stacked relation and moved relative to one another to eliminate the shimmer effect. In the disclosed apparatus, the second diffusion plate reduces the amount of light which is projected on the viewing surface of the projection screen and thereby causes an undesirable reduction in the brightness of the image.
Accordingly, it is an object of the invention to provide an improved projection screen in which the polychromatic shimmer of a projected image is eliminated, without undesirably reducing the brightness of the projected image or moving optical elements of the screen with an undesirably high velocity.
A further object of the invention is to provide such an improved projection screen with at least one Fresnel lens for projecting an image on a diffusion plate, and apparatus for moving the lens relative to the diffusion plate.
Another object of the invention is to provide such an improved projection screen wherein the optical element which projects an image on the diffusion plate is comprised of two cylindrical Fresnel lenses supported in stacked relation and oriented perpendicular to one another.
A further object of the invention is to provide an improved projection screen wherein the optical element which projects the image on a diffusion plate is a cylindrical Fresnel lens having parallel grooves on one face which are oriented perpendicular to corresponding parallel grooves on an opposite face.
Another object of the invention is to provide an improved projection screen wherein the optical element which projects an image on a diffusion plate is a Fresnel lens with a first set of parallel grooves formed in its projection face in perpendicular relation to a second series of parallel grooves formed in the face.
A further object of the invention is to provide an improved projection screen wherein the optical element which projects an image on a diffusion plate is a spherical Fresnel lens.
Another object of the invention is to provide an improved projection screen with a projection lens having a spherically reproducing structure for projecting the image on a diffusion plate, and apparatus for moving the lens relative to the diffusion plate.
A further object of the invention is to provide such an improved projection screen wherein the elements of the projection lens are small prisms.
Another object of the invention is to provide such an improved projection screen wherein the elements of the projection lens are small lenses.
A further object of the invention is to provide an improved projection screen wherein an optical element projects an image on a diffusion plate of the screen and bearing guide elements operate to move the optical element relative to the diffusion plate, to eliminate polychromatic shimmering of the image.
These and other objects of the invention will be better understood by reference to the drawings and the following detailed description of preferred embodiments of the invention.